Chemical plating baths containing an alkali metal cyanoborohydride

ABSTRACT

The invention provides an aqueous plating bath comprising an aqueous solution of a cyanoborohydride of lithium, sodium or potassium and a salt of a metal, such as nickel cobalt, chromium, cadmium, copper and silver, and having a pH between about 3 and about 8. Metals and alloys may be chemically deposited in uniform layers from such plating solutions on substrates which have catalytic surfaces. The term &#39;&#39;&#39;&#39;catalytic surface&#39;&#39;&#39;&#39; as used herein refers to the surface of any article which contains, in whole or part, a material which promotes on its surface the reduction of metal ions.

States I atent [151 3,637,472 Sullivan et all. 1 Jan. 25, 1972 54 CHEMKCAL PLATING BATHS 2,992,885 7/1961 Jackson et al. ..23/35s CONTAINING AN ALKALI METAL g g g il s CYANOBOROHYDRIDE 3 6 Sc e [72] Inventors: Edward A. Sullivan, Beverly; Robert C. Primary Exami"erG- Kaplan Wade, Ipswich, both of Mass. Y-- Harold B055 [73] Assignee: Ventron Corporation, Beverly, Mass. 57 A S RACT [22] Filed: 1970 The invention provides an aqueous plating bath comprising an [21] Appl. 20 075 aqueous solution of a cyanoborohydride of lithium, sodium or potassium and a salt of a metal, such as nickel cobalt, chromium, cadmium, copper and silver, and having a pH between [52] U.S.CI.; ..204/46, 106/1, 117/130, about 3 and about 8. Metals and alloys may be chemically 4/ 204/4 204/50 R, 2 1, 2 R I deposited in uniform layers from such plating solutions on [51] Int. Cl. ..C23b 5/02, C23c 3/02 ub trates which have catalytic surfaces, The term catalytic Field Of Search ,4 surface" as used herein refers to the surface of any article 50 52 52 45 which contains, in whole or part, a material which promotes 359 on its surface the reduction of metal ions.

[56] References Cited 5 Claims, N0 Drawings UNITED STATES PATENTS 2,942,990 6/1960 Sullivan ..106/l CHEMICAL PLATING BATES CONTAHNING AN ALKALI METAL CYANOBOROHYDRIDE The use of chemical reducing agents to plate various metals on different types of substrates by reduction of metal ions in solution has been known for many years. Such processes are called chemical plating to distinguish them from electroplating, in which an electric current reduces dissolved metal ions to plate the elemental metal, such as nickel, cobalt, chromium, cadmium, copper and silver, on surfaces. Among the advantages of chemical plating are (1) its ability'to apply uniform coatings to geometrically complicated surfaces such as the threads of a screw, and (2) its ability to coat materials which are nonconductors of electricity, such as glass, ceramics and plastics, when such materials have been suitably pretreated.

A number of reducing agents have been used successfully in various chemical plating processes, including sodium hypophosphite, formaldehyde, hydrazine, alkali metal borohydrides, and amine-boranes. Some of these reducing agents have general utility while others are restricted to the deposition of particular metals or to more limited use conditions, such as strongly alkaline olutions to stabilize borohydrides as the reducing agent. Similarly, formaldehyde acts as a reducing agent only in alkaline solution.

We have discovered that alkali metal cyanoborohydrides, MBH CN (where M=Li, Na or K), can be used advantageously as reducing agents in chemical plating processes. The most outstanding characteristic of cyanoborohydrides is their unusual stability toward hydrolysis. Aqueous solutions of sodium borohydride, for example, decompose readily by hydrolysis at all ph values below 10. In contrast, aqueous solutions of alkali metal cyanoborohydrides are not subject to hydrolytic decomposition until the pH is less than about 3.

Prior to the present invention, chemical plating baths contained several additives besides the salt of the metal to be deposited and the reducing agent. Most frequently used are complexing agents which, in alkaline solutions, prevent precipitation of the hydroxide of the metal to be plated. In acid solution, complexing agents prevent sludge formation and frequently act as a buffer. A great variety of compounds, usually organic in nature, have been used for these purposes. The net effect of these additives is to improve the stability of plating baths, thereby making them practical.

The present invention provides an aqueous plating bath consisting of an aqueous solution of a cyanoborohydride of an alkali metal selected from lithium, sodium and potassium and a salt of a metal, such as nickel, cobalt, chromium, cadmium, copper and silver, and having a pH between about 3 and about 8. Metals and alloys may be chemically deposited in uniform layers from such plating solutions on substrates which have catalytic surfaces.

The term catalytic surface as used herein refers to the surface of any article which contains, in whole or part, a material which promotes on its surface the reduction of metal ions. Surfaces of glass, ceramics and various plastics are generally noncatalytic. These surfaces can be sensitized to become catalytic by producing a film of one of the catalytic materials on them. This can be done by any of the numerous techniques known to those skilled in the art. A preferred procedure involves dipping articles having noncatalytic surfaces in a solution of stannous chloride and then contacting the treated surface with a solution of palladium chloride, whereby a monolayer of palladium is formed. The article can then be chemically plated by the process of the invention.

The concentration of the alkali metal cyanoborohydride in aqueous solution is important to the plating rate, but operability can be achieved over a wide concentration range from very dilute solutions to saturated solutions. In practice concentrations where the cyanoborohydride is present within the range of 0.01 to about 0.3 g. mole per liter are preferred.

Metal ions may be provided in aqueous solution by adding an appropriate amount of a water-soluble salt such as the chloride, acetate, sulfate, formate, oxalate, etc., of the particular metal. Additional ways of introducing metal ions will occur to those skilled in the art. For example, a metal oxide in the presence of small amounts of sulfuric or hydrochloric acid is a common method used for this purpose.

The concentration of metal ions in solution can be varied over a wide range and is not critical. The plating rate increases slightly with an increase in metal ion concentration. An initial concentration of from 0.02 to 0.05 g. mole per liter of metal salt is preferred.

In carrying out the process of plating, the object to be plated may be prepared by mechanical cleaning, degreasing and acid pickling, according to standard practice in electroplating. The cleaned object is then immersed in a suitable volume of the aqueous plating solution. Within a short time, gas bubbles can be observed forming on the catalytic surface of the object and escaping from the bath in a steady stream, while the article becomes coated slowly with a metallic plate.

The plating process and its rate of accomplishment are influenced by many factors including pH of the plating bath, temperature of the plating bath, concentration of the alkali metal cyanoborohydride, concentration of metal ions, ratio of bath volume to the objects surface area. The plating rate is, in general, increased by an increase in pH or temperature or concentration of alkali metal cyanoborohydride, and to a slight extent by an increase in metal ion concentration. The application of a small electrical potential to the cyanoborohydride plating bath during operation, not greater than about 1.5 volts, has been found to result in higher deposition rates and improved appearance of the plate.

The plating reaction takes place at a uniform rate wherever there is contact between the object being plated and the plating solution. Therefore there is no significant variation in the thickness of deposit even for complicated shapes. There is no buildup on points or edges, and recesses are plated as well as more exposed parts of the object. Such conditions are difficult or impossible to achieve by electroplating. Because of the uniformity of deposit, this process is useful for plating irregularly shaped objects such as fabricated assemblies, parts having holes or screw threads, and the interior surfaces of pipes or vessels.

The purpose of applying the metallic plate may be decorative or functional. In the latter case the function may be protection of the substrate from corrosion, or to provide electrical conductivity to nonmetallic objects, e.g., in printed circuits.

The following examples illustrate various plating baths and conditions under which the process may be carried out. They are in no way intended to limit the scope of this invention.

EXAMPLE l A plating bath was made'from nickel sulfate hexahydrate and sodium cyanoborohydride, having the composition:

NiSO 6H,0 NaBH CN 20 gJl. or 0.076 M 3 g./l. or 0.048 M EXAMPLE 2 Similar plating baths were made from NiSO.,'6l-l 0 and NaBfl CN in which the pH of the solution was adjusted with dilute HCl to values of 3.0, 5.0 and 7.0. Concentrations of nickel ion and cyanoborohydride ion were the same as in example l. Bath temperatures of l60 and 200 F. were used; immersion times were 1 and 2 hours. Deposition of nickel on the steel coupons occurred in all cases. As in example l, higher temperatures and immersion times resulted in heavier deposits. ln appearance, the deposits were brightest at the lowest pH and highest temperature.

EXAMPLE 3 Satisfactory plating results were obtained in plating copper, steel or other catalytic surfaces with aqueous plating solutions of the following composition under the conditions given:

NiCl,-6H,O 0.050.2 mole/liter NaB1-1,CN 0.02-01 mole/liter Temperature l60-200 F.

EXAMPLE 4 To show the contrast between plating baths of the present invention and those of the prior art which usually require complexing agents to control the reaction between the metal ions and the reducing agent, a bath was made of the following composition:

NiS;6H,0 0.076 M Citric acid 0.050 M NaBH CN 0.032 M pH 4.0

When operated for 1 hour at 180 F. with workpieces of steel and copper, no deposit was obtained on either piece. This example demonstrates that, in contrast with usual chemical plating bath compositions, the presence of a metal complexing agent is not only not required but undesirable.

EXAMPLE 5 Deposited plates of cobalt, chromium, cadmium, silver and copper were obtained with cyanoborohydride-containing baths operated at a pH of 4-5 and at l60-l 80 F. for 1 hour, using the following metal salts at the indicated concentrations. In all cases the NaBH CN concentration was 0.05M.

The application of a small electrical potential to the cyanoborohydride plating bath during operation was found to result in higher deposition rates and improved appearance of the plate. Baths of the following composition were used Nisommo 0.08 M NaBlhCN 0.05 M pH 50 Copper workpieces were used as the cathode and a steel strip as the anode.

Varying potentials were applied using a Sargent-Slomin Electrolytic Analyzer. After 30 minutes in the plating bath, the workpieces were removed, rinsed, dried and evaluated. Deposition rates and variable conditions are given below:

With cyanobnrohydrlde Tcmperatu re Potential Deposition rate (F.) (DC volt!) (mg/cm!) Identical tests were performed except that the baths contained only the nickel salt; no cyanoborohydride was present. The results are given in the following table:

Without cyanoborohydride Temperature Potential Deposition rate (F.) (DC volts) (mg/emf) 70 1.00 0 70 1.25 0 70 1.75 0

We claim:

1. A plating bath for chemically plating metals selected from the group consisting of nickel, cobalt, chromium, cadmium, copper, and silver, upon a catalytic surface, said bath consisting of an aqueous solution of a cyanoborohydride of an alkali metal selected from the group consisting of sodium, potassium and lithium, in an amount sufficient to act as a reducing agent for the plating metal, and a salt of said plating metal, said aqueous solution having a pH between about 3 and about 8.

2. A plating bath as claimed by claim 1 wherein the concentration of said alkali metal cyanoborohydride is between about 0.01 and about 0.3 moles per liter of said aqueous solution.

3. A method for chemically plating a metal selected from the group consisting of nickel, cobalt, chromium, cadmium, copper, and silver upon a substrate having a catalytic surface which comprises immersing said substrate in an aqueous bath comprising an aqueous solution of a salt of a metal selected from said group and a cyanoborohydride of an alkali metal selected from the group consisting of sodium, potassium, and lithium, said aqueous solution having a pH between about 3 and about 8.

4. The method as claimed by claim 3 wherein the concentration of said alkali metal cyanoborohydride is between about 0.01 and about 0.3 moles per liter of said aqueous solution.

5. The method as claimed by claim 3 wherein the rate of deposition is increased by immersing an anode and said substrate as a cathode in said aqueous bath and imposing a direct current potential not greater than about 1.5 volts therebetween. 

2. A plating bath as claimed by claim 1 wherein the concentration of said alkali Metal cyanoborohydride is between about 0.01 and about 0.3 moles per liter of said aqueous solution.
 3. A method for chemically plating a metal selected from the group consisting of nickel, cobalt, chromium, cadmium, copper, and silver upon a substrate having a catalytic surface which comprises immersing said substrate in an aqueous bath comprising an aqueous solution of a salt of a metal selected from said group and a cyanoborohydride of an alkali metal selected from the group consisting of sodium, potassium, and lithium, said aqueous solution having a pH between about 3 and about
 8. 4. The method as claimed by claim 3 wherein the concentration of said alkali metal cyanoborohydride is between about 0.01 and about 0.3 moles per liter of said aqueous solution.
 5. The method as claimed by claim 3 wherein the rate of deposition is increased by immersing an anode and said substrate as a cathode in said aqueous bath and imposing a direct current potential not greater than about 1.5 volts therebetween. 